1. Field of the Invention
The present invention relates to a thermochromic window and a method of fabricating the same, and more particularly, to a thermochromic window, the sunlight transmittance of which is adjustable depending on temperature, and a method of fabricating the same.
2. Description of Related Art
In response to increasing prices of chemical energy sources such as petroleum, the necessity for the development of new energy sources is increasing. In addition, the importance of energy saving technologies is increasing with the necessity for these new energy sources. In fact, at least 60% of energy consumption in common houses is attributed to heating and/or cooling. In particular, common houses and buildings lose as much as 24% of their energy through windows.
Accordingly, a variety of attempts have been made to reduce the amount of energy that is lost through windows by increasing the airtightness and insulation characteristics thereof, while maintaining the aesthetics and characteristics of the view which are the basic functions of windows. Typical methods, by way of example, include varying the size of the window and furnishing highly-insulated windows.
Types of highly-insulated window glass include argon (Ar) injected pair-glass in which Ar gas is situated between a pair of glass panes in order to prevent heat exchange, a vacuum window in which the air between a pair of glass panes is evacuated, a low emissivity (low-e) window, and the like. Also being studied is a type of glass that is coated with a layer that has specific thermal characteristics in order to adjust the amount of solar energy that is introduced.
In particular, the surface of the low-e window is coated with a thin layer of metal or metal oxide which allows most visible light that is incident on the window to enter so that the interior of a room can be kept bright, while radiation in the infrared (IR) range can be blocked. The effects of this glass are that it prevents heat from escaping to the outside when heating in winter, and also prevents heat energy from outside a building from entering in summer, thereby reducing cooling and heating bills. However, this window has the following drawbacks due to its characteristic of reflecting wavelengths other than visible light. Specifically, it does not admit the IR range of sunlight into a room, which is a drawback, especially in winter, and the sunlight transmittance thereof is not adjusted according to the season (temperature).
Accordingly, a variety of technologies for thermochromic windows which are provided by coating glass with a thermochromic material is being developed. Such a thermochromic window blocks near infrared (NIR) radiation and infrared (IR) radiation while allowing visible light to pass through when the glass arrives at a predetermined temperature or higher, thereby preventing the room temperature from rising. This can consequently improve cooling/heating energy efficiency.
In particular, a variety of studies are underway on thermochromic windows which are created by coating glass with vanadium dioxide (VO2). The phase transition temperature of VO2 is 68° C., which is close to the temperature at which practical application is possible. In addition, it is easy to control the transmittance of VO2 since its optical constant (n, k) changes significantly.
However, such a thermochromic thin film disadvantageously has a reflective color of dark yellow and a low visible light transmittance caused by a high absorption coefficient for short wavelengths.
Accordingly, in order to increase the visible light transmittance of the thermochromic window, an antireflection layer is formed on the thermochromic window or the thermochromic thin film is patterned by photolithography.
FIG. 1 is a graph showing variations in the transmittance of two thermochromic windows depending on the wavelengths before phase transition (at 20° C.) and after the phase transition (at 90° C.). One thermochromic window has a VO2 thin film coating a glass substrate, and the other thermochromic window has antireflection films formed by respectively disposing Al2O3 and TiO2 thin films on the upper and lower surfaces of a VO2 thin film formed on a glass substrate.
As shown in FIG. 1, it is apparent that the transmittance of the thermochromic window having the antireflection films is increased in the whole visible light range. While the visible light transmittance is increased, the transmittance in the infrared (IR) range is significantly decreased. The decreased IR transmittance deteriorates the transformation characteristics of the thermochromic window (variations in the IR transmittance before and after phase transition).
In addition, the antireflection film has a multilayer structure including a high refractive index thin film and a low refractive index thin film which are stacked on each other. This type of deposit film is disadvantageous in that its fabrication process is complicated.
Furthermore, the approach of increasing the transmittance of the thermochromic window by patterning the thermochromic thin film is disadvantageous in that its process is complicated and expensive.
The information disclosed in the Background of the Invention section is provided only for better understanding of the background of the invention, and should not be taken as an acknowledgment or any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.